Most plastic bottle used for a packaging container has an outer thread at the neck portion and a cap is screwed on the thread to seal the opening portion. This outer thread can be formed by using a split mold, so there is no technical hardness in the injection molding. However, in the case of an inner thread, it is transferred from an outer thread pattern formed on the outer surface of a core mold. Therefore, the core should be pulled out rotating it after molding.
This pulling-out of the core is performed not only by rotating the core but also by moving the core in the axial direction. Therefore, the rotation and the axial moving should be synchronized. However, the synchronizing action mechanically limits the number of products per one molding. Thus, it is difficult to mold many products at one time like a preform molding because of a mechanical restriction. For this reason, even if a screw-plug is suitable for sealing the opening portion of a certain packaging container, a screw cap sealing with an outer thread is used to avoid difficulty of forming an inner thread.
The object of the present invention is to provide a novel method for forming an inner thread at the neck portion of the preform, the method enabling an injection core whose base portion forms an inner thread being pulled out after molding by enlarging the neck portion by blowing air, which has been desired in the molding of the preform for a packaging container such as a bottle having an inner thread at the neck portion thereof.